Couple of things.  Depends on the accuracy of what you are trying to
accomplish, but I've done what Mark suggested, that is to use a PIC to
'count' 50/60 Hz cycles, and as long as your sample clock is fast enough to
poll for the square wave and update the counter, it works rather well.

I had the same problem with a EXAR chip that went obsolete.  XR2240,
programmed via dip switches, basically it was a counter that could be
reloaded at the end of its cycle, or act as a one-shot. Gee...after 10+
years in a design, it finally went obsolete.

I ended up doing this one in an ALTERA 7xxx series so I could put all 3 of
them in one package, then did a little daughter board for it.